The manufacture of paper typically involves the processing of a carefully prepared aqueous fiber suspension to produce a highly uniform dry paper sheet. Three steps included in the typical process are sheet forming, where the suspension is directed over a porous mesh or "wire" upon which fibers are deposited while liquid filters through the wire, sheet pressing, where the formed sheet is passed through presses covered with porous "felt" to extract retained water from the sheet, to improve the sheet's uniformity, and to impart surface quality to sheet; and paper drying, where residual water is evaporated from the sheet. The sheet may then be further processed into the finished paper product.
It is well known that evaporation of water is energy intensive and thus relatively expensive. Consequently, efficient papermaking is dependent upon extracting water during the forming and pressing operations, and avoiding sheet defects which render the dried sheet unfit for use. Felts and wires are thus particularly important because they affect not only water removal but, because of their intimate contact with the sheet, the quality of the sheet itself. Deposits allowed to collect on the felt or wire can affect its water removal efficiency, can cause holes in the sheet, and can be transferred to the sheet material to create defects.
The quality of the aqueous fiber suspension used to produce the sheet is dependent upon many factors, including the wood and water used as raw materials, the composition of any recycled material added to the process, and the additives used during preparation of the suspension. Thus a variety of dissolved or suspended materials can be introduced into the manufacturing process, including both inorganic materials such as salts and clays, and materials which are organic in nature such as resins (also referred to as "pitch"), fine cellulosic fibers and other particles from the wood, as well as inks, latex, and adhesives from recycled paper products. A build up of deposits containing inorganic and/or organic materials on felts and other sheet forming equipment during the manufacturing process is recognized as a troublesome obstacle to efficient papermaking. Particularly troublesome are the sticky materials such as glues, resins, gums and the like which are associated with recycled fibers.
Methods of quickly and effectively removing deposits from the papermill sheet forming equipment are of great importance to the industry. The paper machines could be shut down for cleaning, but ceasing operation for cleaning is undesirable because of the consequential loss of productivity. On-line cleaning is thus greatly preferred where it can be effectively practiced.
The wire belt or cylinder used for sheet forming cycles continuously, as a belt, during production. The sheet-contact portion of the cycle begins where application of the fiber suspension to the wire belt or cylinder is started and continues until the formed sheet is separated from the wire surface; and the return portion of the cycle returns the wire from the position where the formed sheet has been removed from its surface to the beginning of the sheet-contact portion. With wire belts such as Fourdrinier wires, on-line wire cleaning has generally been performed during.the return stage (i.e. where the wire is not in contact with the forming sheet) by treating the returning wire with a cleaning liquid (typically water). Often the wire is showered with liquid under pressure, which may be further assisted by mechanical surface cleaning. Use of water showers, with or without mechanical assistance, has not proved entirely satisfactory in removing a build-up of either organic compounds or inorganic deposits on the wires, and additional materials have been used to provide cleaning liquids which are more effective. Predominantly fibrous or inorganic materials have been successfully removed using water-based formulations containing either acids or alkalis formulated with other chemicals such as surfactants. Where organic deposits are prevalent, they have been removed with some success by using organic solvents, including some formulations containing aromatic compounds with low flash points or chlorinated hydrocarbons. In most machines polyester fabric belts are now used instead of the more traditional wires.
Papermill felts also commonly circulate continuously in belt-like fashion between a sheet contact stage and a return stage. During the sheet contact stage water is drawn from the sheet usually with the aid of presses and/or vacuum into the pores of the felt. A clean felt, having fine pores which are relatively open, is especially desirable for effective paper manufacture since this allows efficient removal of water from the paper sheet. A felt cleaning procedure should remove both organic and inorganic deposits of both a general and localized nature, maintain felt porosity, and condition the fabric nap without chemical or physical attack on the substrate. However, cleaning liquids are also utilized to remove troublesome build-up of organic and inorganic deposits. The fabric composition and conformation of many papermill felts makes them susceptible to chemical degradation. The cleaning chemicals should be easily removed by rinsing. Both continuous and shock cleaning is used in most papermills. The chemicals used include organic solvents, often chlorinated hydrocarbons. Acid and alkali based systems are also used, but at lower concentrations than used in wire cleaning. High concentrations of alkali metal hydroxides are often unsuitable for felt cleaning as they "attack" the fabric material.
Some of the more successful organic solvents have been identified as health risks, such as carcinogens, and thus require especially careful handling. Other solvent based products can damage plastic or rubber components used in the paper forming process. One on-line treatment of felts which we know has been used for several years with some success involves contacting the felt with aqueous solution of cationic surfactants such as alkyldimethyl benzyl ammonium chloride wherein the alkyl group consists of a mixture of C.sub.12 H.sub.25, C.sub.14 H.sub.29 and C.sub.16 H.sub.33 groups. However, experience has shown that some sticky materials will tend to adhere to felts despite treatment with these surfactants. Another felt conditioning practice which has been advocated in the past is application of aqueous solutions of cationic polymers to the felts. However this type of treatment can actually lead to a build-up of deposit of materials derived from the cationic polymers themselves.
Other sheet forming equipment such as deckers, filters, screens, and rolls can also become fouled. The process problems and treatments are, as a general rule, similar to the felt system, although certain considerations such as maintaining porosity and avoiding chemical degradation of fabric, which are important in felt cleaning and cleaning certain other fine-pored equipment components, may not be so critical for this other equipment.
Natural resin or gum in fresh wood can vary, depending on the species. Some types of pine wood, especially those containing 2 weight percent or more of resin, are commonly used in only very low percentages due to the gum and resin problems they cause. Papermakers alum or sodium aluminate have been traditionally used to control natural wood resin deposits. These products are added into the total pulp system with the objective of depositing the resin on the fiber. The effectiveness of this approach is limited by such factors as pH, the potential for corrosion, paper sheet formation, and the need to control interaction with other chemicals in the pulp system. Treatments which would permit the unrestricted use of these problem pine wood sources could have significant beneficial economic impact on some pulp and paper producers.
The increasingly more common use of recycled fiber has contributed to more serious build-ups of sticky material during paper formation. The glues, resins, gums, etc. which are found in recycled, secondary fiber tend to adhere to various parts of the paper-forming machine and to resist on-line shower cleaning. The materials which adhere to the felt can seriously affect drainage and paper formation. The end result in the product is holes, and ultimately, in some cases, breaks in the sheet during paper processing. Frequent shutdown may be necessary to solvent wash the felt to remove the particularly sticky material associated with recycled fiber. The advantages of paper recycling can thus be somewhat offset by reduced productivity of the papermaking machines.
EPA No. 279,089, for example, discloses the use of such organic cleaners with an alkali in both water and an organic cosolvent.
Another approach to deposit control has been the use of pulp additives such as anionic aryl sulfonic acidformaldehyde condensates or cationic dicyandiamide-formaldehyde condensates. The additives may function for example as sequestrants, dispersing agents or surface active agents. In particular the cationic dicyandiamide-formaldehyde aminoplast resins have been described as bringing about the attachment of pitch (e.g. resinous matter and gums), in the form of discrete particles, to pulp fibers so that the pitch particles are uniformly distributed on the fibers themselves. Consequently, the amount of pitch which accumulates on the papermaking machine is reportedly reduced without causing dark spots or specks of pitch in the paper product.
U.S. Pat. No. 4,995,944 discloses that the deposit of sticky material from papermaking pulp onto papermill felts and other papermaking equipment used in processing a pulp slurry into sheets can be inhibited by applying to the equipment an aqueous solution containing 2 ppm of a cationic polymer and applying to the equipment an aqueous solution containing compounds selected from the group consisting of water-soluble non-ionic and cationic surfactants in an amount effective to inhibit build-up of deposits derived from the cationic polymer. This technique is said to be particularly beneficial when used for treating felts and like equipment components used in processing pulp slurry into sheets.
Especially in the case of wet press felts, chemical treatments such as caustic cleaning (with NaOH) or detergents are performed to weaken and remove deposits by attacking the oily or resinous components of the deposit, both on a continuous basis and with greater intensity when the machine is shut down for periodic maintenance. In addition, mechanical loosening and removal of the deposits is accomplished with the use of high pressure oscillating water showers and vacuum devices which can remove water and contaminants while the machine is in operation. The deposits are composed of inert filler materials, cellulosic components, and resinous or polymeric components which bind the other components together and into the fibrous structure of the felt. These deposits fill up the void spaces of the felt reducing porosity, hence the capability of the felt to handle water during the pressing operation. The reduction of water handling ability can reduce speed and efficiency of the paper making operation to the point where eventually the felt must be removed and replaced. In addition, nonuniformity in the build up or removal of the deposits can induce variations in the ability of the press to handle water resulting in nonuniformity of the moisture content of the paper or in operating difficulties of the paper machine.
In addition to the cleaning agents mentioned above, enzymatic preparations have been considered for the processing of pulp associated with conventional chlorine bleaching steps, e.g., WO91/02839, which discloses using xylanases, cellulases and hemicellases as a pre-treatment step before the first chlorination stage to reduce the active amount of chlorine used.
WO92/16687 discloses treating the pulp or "white water" with an enzymatic preparation, which is said to reduce pitch problems associated with mechanical pulp and/or papermaking pulp containing same. More particularly, the enzymes are said to be added at any pulp production stage after the mechanical detachment of fibers. Although this reference states that such treatment improved machine runnability and also that the felts appeared clean, use of the enzymatic preparation was limited to only the wet stage of the papermaking process. Once the paper web is transferred from the forming wires of the Fourndrier to the press felts, most of the water containing the enzyme preparation has already been driven-off. Effectiveness of this method depends upon the amount of residual enzyme available in the water passing through the press felts. In the press section referred to here as the dry stage of the machine, which is not addressed by this approach, there is negligible residual enzyme available, moreover, no new enzymatic treatment is introduced.